1. Field of the Invention
This invention relates to a semiconductor memory device and a test method thereof and, for example, a semiconductor memory device using magnetoresistive elements which can record information by causing currents to flow in both directions therethrough.
2. Description of the Related Art
Recently, various types of memories which record information based on a new principle are proposed and, as one of the above memories, a magnetic random access memory (MRAM) utilizing a tunneling magnetoresistive (TMR) effect is known.
The MRAM stores “1”, “0” information by means of a magnetic tunnel junction (MTJ) element. The MTJ element has a structure formed by disposing a non-magnetic layer (tunnel barrier layer) disposed between two magnetic layers (free layer and pinned layer). Information stored in the MTJ element is determined according to whether the directions of spins of the two magnetic layers are parallel or anti-parallel.
In a spin transfer torque writing type MRAM, inversion of magnetization occurs by causing a current to flow in a direction perpendicular to the film surface of the MTJ element to transfer spins into the free layer according to the flow direction of the current. When the MTJ element is of the perpendicular magnetization type, it is sufficient to provide a uniaxial anisotropy in the direction perpendicular to the film surface and it is unnecessary to provide a magnetic shape anisotropy in the plane direction unlike the case of the in-plane magnetization type. Therefore, it becomes possible to set the aspect ratio of the MTJ element to approximately 1 and reduce the size of the MTJ element to the processing limitation in principle. Further, it becomes unnecessary to provide a current-induced magnetic field wiring which generates current-induced magnetic fields in directions on two axes unlike the in-plane magnetization type. Since the operation can be performed if two terminals connected to upper and lower electrodes of the MTJ element are present, the cell area for each bit can be reduced.
Recently, a film of polysilicon magnesium oxide (MgO) oriented in a (001) plane as a tunnel barrier layer of the MTJ element is disposed between films of polysilicon CoFeB oriented in a (001) plane so as to cause MgO to act as a spin filter. Magnetization of the free layer can be inverted from anti-parallel to parallel by injecting electrons from the pinned layer to the free layer, magnetization of the free layer can be inverted from parallel to anti-parallel by injecting electrons from the free layer to the pinned layer and it is confirmed which magnesium oxide (MgO) is used as a preferable material which realizes a spin transfer torque writing type MRAM having high TMR.
In order to reduce a write current, it is necessary to reduce the volume of the free layer, saturation magnetization Ms, damping constant and the like. However, there is a physical limit on a reduction in the film thickness to reduce the volume of the free layer, there is a processing limit on a reduction in the area in the plane direction and thermal stability is degraded if the damping constant is excessively reduced. Therefore, it becomes necessary to adjust parameters so as to attain balance as a whole and reduce a write current and it is not easy to reduce the above factors. When a write current cannot be sufficiently reduced, it becomes necessary to set a desired write current by reducing the film thickness of the MgO barrier to lower the resistance thereof since the power source voltage of the circuit is normally determined. Therefore, the MgO barrier which is a constituent of the MTJ element is required to be sufficiently thin and is kept applied with high-voltage stress during the operation.
In the case of an MTJ laminated film using MgO, since the magnetoresistive ratio (MR ratio) exceeds 100% even if it is formed of a thin film, the resistance in the “1” state is set to a value which is approximately equal to or twice the resistance in the “0” state. Therefore, it is estimated that total stress applied to the film becomes greatly different when the MTJ element is set in the “0” state and a write current of “0” is caused to flow continuously and when the MTJ element is set in the “1” state and a write current of “1” is caused to flow continuously. When the stress applied thus becomes greatly different, the service life of the MTJ element becomes different.
For example, when the MRAM is used in a drive recorder as an image memory, it is estimated that there occurs a case wherein “0” information is intensively used and recorded in the MTJ element or a case wherein “1” information is more intensively recorded than “0” information depending on the condition in the application environment or the configuration of a decoder for image information. Therefore, since the service life of the MTJ element becomes different depending on the application environment, the device must be designed to cope with the worst environment as a specification of the device. Therefore, there occurs a problem that the resultant device specification must be severely set.
As the technique associated with the above technique, the following patent document is provided.
Jpn. Pat. Appln. KOKAI Publication No. 2002-343078